System and method for monitoring electrosurgical systems

ABSTRACT

Systems, apparatus, and methods for monitoring electrosurgical systems are disclosed. In one variation, an electrosurgical monitoring apparatus includes at least two monitoring channels. Each of the monitoring channels in this embodiment are configured to monitor fault current between at least two separate conductive components of an electrosurgical system, and the at least two monitoring channels are each configured to send a control signal so as to control an application of power to an electrosurgical device responsive to the fault currents.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.11/202,605, now abandoned filed Aug. 12, 2005, entitled SYSTEM ANDMETHOD FOR MONITORING ELECTROSURGICAL SYSTEMS, which claims priorityfrom commonly owned and assigned provisional application No. 60/602,103,entitled SYSTEM FOR MONITORING RESECTOSCOPES AND RELATED ELECTROSURGICALINSTRUMENTS, filed Aug. 17, 2004, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to surgical techniques and devices. Inparticular, but not by way of limitation, the present invention relatesto electrosurgical techniques.

BACKGROUND OF THE INVENTION

The problems arising with the use of electrosurgical instruments wherethe field of view of the surgeon is limited are well-known. Traditionallaparoscopic electrosurgical tools include a trocar sheath or othercannula that is inserted into a patient's body and that provides aconduit for a surgeon to introduce various surgical cutting tools,optics for increased visualization, irrigation, active surgicalelectrodes, and other devices to be used during a surgical procedure.

One problem arises if the insulation on the active electrode is damagedthereby allowing the active current (possibly in the form of arcing) topass there-through directly to the patient's tissue whereby unintendedand potentially unknown injury, possibly in the form of a lifethreatening infection, can occur. The arcing may occur out of thesurgeon's field of view which may extend as little as about 2centimeters from the tip of the active electrode (or the surgicalfield). The field of view is typically established by illumination andviewing sources. In the context of prior art laparoscopic instruments,the illumination and/or viewing sources are established through one ormore other trocar sheaths at other incisions.

Particularly with electrosurgical instruments, there can be manycentimeters of the active electrode which extend between the entry pointin a patient's body and the surgeon's field of view, typically at thedistal end of the active electrode and near the point whereelectrosurgery takes place. The area of the electrosurgical instrument,and in particular the active electrode, that is out of the field of viewof the surgeon is potentially dangerous if left in an unmonitored state.In this situation, the insulated active electrode may unintentionallycome into contact with unknown tissue of the patient may cause seriousinjury that might not be noticed by the surgeon during the procedure.

If arcing resulting from the damaged insulation were to occur within thefield of view of the surgeon, the surgeon would normally observe thisand immediately deactivate the generator. Arcing, however, is prone tooccur at a site remote from the field of view of the surgeon, and as aconsequence, damage to the active electrode insulation is particularly aproblem because it may go undetected while the full active currentpasses through an unintended path of the patient's tissue from theactive electrode to the return electrode.

A second problem that can arise is caused by a capacitive effect whereone electrode of the capacitance is the active electrode and the otherelectrode of the capacitance is the metallic trocar sheath. Thedielectric between these elements is the insulation on the activeelectrode. Current from the active electrode will be capacitivelycoupled to the trocar sheath and then returned through the body and thereturn electrode to the generator. If this current becomes concentrated,for example, between the trocar sheath and an organ such as the bowel,the capacitive current can cause a burn to the organ.

With respect to the use of laparoscopic electrosurgical tools, the aboveproblems have been preliminary addressed by the use of a safety shieldand/or monitoring circuitry which serves to deactivate theelectrosurgical generator and accompanying current flow if an abnormalcondition occurs. For example, U.S. Pat. Nos. 5,312,401, 5,688,269,5,769,841 and 6,494,877, assigned to Encision, Inc., describe solutionsto these problems. All of the details of these patents are herebyincorporated into the present application by reference in theirentirety.

U.S. Pat. No. 4,184,492, by Meinke, discloses, in general, a system inwhich a resecting apparatus includes a connection between an outer tube(metallic) and a lead means (the return electrode) with an impedance of100-1000 ohms. The purpose is to minimize or avoid burns to the patientand user touching the metallic parts of the instrument. There may be amonitor included in the connections to display unsafe conditions andalso reduce power.

The assignee of the Meinke patent, Karl Storz Endoscopy-America, Inc.,has not, to this day, offered a monitored or otherwise protectedresectoscope that embodies the description contained in the Meinkepatent indicating that there were, and continue to be, significanthurdles in the implementation of such a monitored or protected system ina resectoscopic device. The complex design issues of modernresectoscopes and associated surgical techniques have not changedsignificantly since the Meinke patent and the same problems describedtherein persist today.

It is thus desirable to overcome the inherent problems associated withincorporating the use of shielded and/or monitored systems such as thosedisclosed in the prior art into devices such as resectoscopes andhysteroscopes and to give the same, or better, level of protection topatients that is achieved with those prior systems.

Conventional resectoscopes, such as those manufactured by Karl Storz,combine many features into a single device. Such devices are typical ofthe devices that are predominantly used in many urological andgynecological electrosurgical procedures. It is estimated thatapproximately 200,000 of the resectoscopic surgeries in the UnitedStates alone are performed with a Storz instrument. This representsapproximately ⅔ of the total procedures performed each year. U.S. Pat.No. 6,755,826, assigned to Olympus, gives one example of some of themechanical complexities of a resectoscope. The details of the '826patent are hereby incorporated by reference into this disclosure intheir entirety.

Resectoscopes, such as those manufactured by Karl Storz, involve complexmechanics and generally bulky construction when compared withlaparoscopic devices. For example, resectoscopes employ many components,each of which must be used in combination in a single device. Inlaparoscopic procedures, several separate devices are typically used toperform the many functions of a resectoscope. These include optics,illumination, irrigation (both in and out), electrical function (RFpower), and the mechanical linkages for operation of the cutting tools.In addition, user proximity to resectoscopic devices presents its ownchallenges and increased need to prevent current from energizing thecomponents that are near the surgeons face. Since the surgeon's face is,in many situations can be close to metallic conductive optics, there isthe potential for current to flow directly to the surgeon and causeinjury.

There are several additional problems that need to be overcome inresectoscopic and like devices that are not addressed in the prior artand that have not been addressed in any currently available technology.For example, the 100 ohm impedance addressed in the Meinke '492 patentis not low enough to completely and/or adequately couple the harmfulcurrent away from the patient and the user. While it does cut down thecurrent flow, it is not adequate for shunting fault currents through thereturn electrode, particularly in applications where instruments havemetallic components (e.g., resectoscopic applications). The 100 ohmimpedance disclosed in the Meinke '492 patent is meant to preventalternate return current from flowing through the metal components tothe generator return. 100 ohms is not high enough to do that completelyand some portion of the total current could still be conducted and maybe enough to cause a burn at the contact with wet tissue.

Finally, resectoscopes are subject to otherwise “normal” working elementcurrent surges due to blood and/or other conductive fluid tissuebridging the working element and active electrode. These current surgesare normally present on only a temporary basis and may or may notrepresent a dangerous condition to the patient that requiresintervention.

Although present devices are functional, they are not sufficientlyaccurate or otherwise satisfactory. Accordingly, an improved system andmethod are needed to address one or more of the various shortfalls ofpresent technology and to provide other new and innovative features.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention that are shown in thedrawings are summarized below. These and other embodiments are morefully described in the Detailed Description section. It is to beunderstood, however, that there is no intention to limit the inventionto the forms described in this Summary of the Invention or in theDetailed Description. One skilled in the art can recognize that thereare numerous modifications, equivalents and alternative constructionsthat fall within the spirit and scope of the invention as expressed inthe claims.

In one variation, the invention may be characterized as anelectrosurgical monitoring apparatus that includes at least twomonitoring channels. Each of the monitoring channels in this embodimentare configured to monitor fault current between at least two separateconductive components of an electrosurgical system, and the at least twomonitoring channels are each configured to send a control signal so asto control an application of power to an electrosurgical deviceresponsive to the fault currents.

In another variation, the invention may be characterized as anelectrosurgical system that includes a first conductive surgical systemcomponent, a second conductive surgical system component, anelectrosurgical tool, and a monitor. The monitor in this embodimentincludes at least two monitoring channels, a first of the monitoringchannels monitors fault current flowing through the first conductivesurgical system component, and a second of the monitoring channelsmonitors fault current flowing through the second conductive surgicalsystem component, and the at least two monitoring channels are eachconfigured to send a control signal so as to control an application ofpower to the electrosurgical tool responsive to the corresponding faultcurrent.

As previously stated, the above-described embodiments andimplementations are for illustration purposes only. Numerous otherembodiments, implementations, and details of the invention are easilyrecognized by those of skill in the art from the following descriptionsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects and advantages and a more complete understanding of thepresent invention are apparent and more readily appreciated by referenceto the following Detailed Description and to the appended claims whentaken in conjunction with the accompanying Drawings wherein:

FIGS. 1 and 2 are block diagrams depicting one embodiment of a systemfor monitoring an electrosurgical procedure;

FIG. 3 is a block diagram depicting another embodiment of a system formonitoring an electrosurgical procedure;

FIG. 4 is a block diagram depicting yet another embodiment of a systemfor monitoring an electrosurgical procedure;

FIG. 5 is a flowchart depicting steps carried out in accordance with anelectrosurgical procedure;

FIG. 6 is a block diagram depicting one embodiment of a system formonitoring an electrosurgical procedure in which the reference potentialdepicted in FIG. 4 is derived from a voltage of a patient;

FIG. 7 is a block diagram depicting an embodiment of another system formonitoring an electrosurgical procedure in which the reference potentialdepicted in FIG. 4 is generated;

FIG. 8 is a block diagram depicting a variation of the system depictedin FIG. 7;

FIG. 9 is a flowchart depicting steps carried out in connection withpreparing an electrosurgical apparatus for an electrosurgical procedurein accordance with the embodiment depicted in FIGS. 7 and 8;

FIG. 10 is a cross sectional view of an exemplary electrode assembly;

FIG. 11 is a block diagram of a system for monitoring both a conductivebody and a shield of the electrode assembly depicted in FIG. 10;

FIG. 12 is a schematic representation of a resectoscope that may be usedin connection with the embodiments disclosed with reference to FIGS.1-9;

FIGS. 13A, 13B and 13C depict respective front, top and cross sectionalviews of an exemplary embodiment of a resectoscope that may be used inthe embodiments disclosed with reference to FIGS. 1-11;

FIG. 14 a perspective view of the resectoscope depicted in FIG. 13 in adisassembled form; and

FIGS. 15A and 15B are a cross sectional and a front views of a portionof the resectoscope depicted in FIGS. 13 and 14.

DETAILED DESCRIPTION

Referring now to the drawings, where like or similar elements aredesignated with identical reference numerals throughout the severalviews, FIG. 1 illustrates a block diagram of one embodiment of a system100 for monitoring an electrosurgical procedure. As shown, a generator102 is coupled to an active electrode 104 via an active line 106, and aconductive body 108 that supports the active electrode 104 is showncoupled to a return electrode 110 of the generator 102 via a lowimpedance path 112.

As depicted in FIG. 1, the low impedance path 112 includes a monitor 114that is coupled to the generator 102 with a control line 116. The activeelectrode 104 and the return electrode 110 are shown contacting apatient 118 so as to create a normal current path that is shown runningfrom the generator 102, through the active line 106, active electrode104, the patient 118 and the return electrode 110 back to the generator102. Also shown is a fault current path that runs from a portion of theactive electrode 104, through the conductive body 108 and the lowimpedance path 112.

The generator 102 in the exemplary embodiment is a high frequencyelectrosurgical generator capable of generating radio frequency currentin the range of 50 KHz to 5 MHz, but the type of generator implementedmay vary depending upon the type of electrosurgical procedure beingperformed. Examples of high frequency generators include the ERBE ICC350 electrosurgical generator available from ERBE Elektromedizin,Tubingen, Germany and the FORCE-2 and FX electrosurgical generatorsavailable from VALLEYLAB of Boulder, Colo.

The monitor 114 in this embodiment is implemented with a low impedancemonitor configured to measure current and/or voltage in the faultcurrent path. The impedance of the monitor 114 in several embodiments issubstantially less than 100 Ohms, and in other embodiments the impedanceis less than or equal to about 50 Ohms. In yet other embodiments, theimpedance of the monitor 114 is less than or equal to about 30 Ohms. Anexemplary monitor that has an impedance of about 20 Ohms is an EM-2style monitor manufactured by Encision, Inc of Boulder Colo., but it iscontemplated that the monitor 114 may be implemented with an impedanceof less than or equal to about 15 Ohms.

In several embodiments, the conductive body 108 is a portion of anelectrosurgical apparatus (e.g., an endoscope) that is not intended toimpart surgical-level voltages (e.g., voltages that ablate tissue) tothe patient; yet the conductive body 108 is susceptible (or evenintended) to contact the patient during an electrosurgical procedure. Inmany embodiments, the conductive body 108 provides mechanical supportfor elements of the surgical apparatus. For example, the activeelectrode 104 is generally supported, yet electrically insulated from,the conductive body 108. Although conductive bodies of surgicalinstruments are frequently discussed herein in the context of endoscopesfor exemplary purposes, it should be recognized that the conductivebodies described herein may be realized in a variety of electrosurgicalapparatus including endoscopes, colonoscopies, laproscopic instrumentsand catheter systems.

The active electrode 104 in the exemplary embodiment imparts a voltage,also referred to herein as an electrical potential, generated by thegenerator 102 to the patient 118. In some embodiments 104 the activeelectrode 104 includes a rollberball configuration and in otherembodiments a cutting loop, but other embodiments are certainlycontemplated and are well within the scope of the present invention.

In operation, when a potential is applied to the patient 118 with theactive electrode 104, a current, following the normal current path,flows from the active electrode 104, through the patient 118 to thereturn electrode 110, which is coupled to another portion of the patient118. In several embodiments, the current alters (e.g., ablates) tissueof the patient 118 that is within and around the normal current path soas to effectuate a surgical procedure.

During an electrosurgical procedure, one or more events can cause thepotential of the conductive body 108 to approach the potential of theactive electrode 104. For example, if the insulation surrounding theactive electrode 104 fails, the impedance between the conductive body108 and the active electrode 104 will decrease and may allow current toflow (e.g., arc) from the active electrode 104 to the conductive body108. In addition, during some electrosurgical procedures, conductivefluids and/or tissue are prone to accumulate between the activeelectrode 104 and the conductive body 108. These conductive fluids alsoreduce the impedance between the active electrode 104 and the conductivebody 108, which allows current to flow, via the conductive fluid, fromthe active electrode 104 to the conductive body 108.

In accordance with several embodiments of the present invention, the lowimpedance path 112 effectively shunts current from the conductive body108 to the return electrode 110 so as to prevent the conductive body 108from reaching a much higher potential. In this way, the conductive body108 is prevented from attaining a level of potential that wouldotherwise be harmful to the patient. In some embodiments where theconductive body 108 forms part of an endoscope for example, portions ofthe conductive body 108, telescope (not shown), and sheath (not shown)routinely contact the patient, and if current is not shunted away fromthe patient 118, current from these portions of the surgical tool canseverely burn the patient 118 at unintended locations.

As depicted in FIG. 1, when an event occurs that causes the potential ofthe conductive body 108 to approach that of the active electrode 104,fault current following the fault path flows from the conductive body108 through the low resistance path 112 to the return electrode 110. Inaccordance with several embodiments of the present invention, the lowimpedance path 112 has an impedance that is substantially less than 100Ohms.

Creating a low impedance path between the conductive body 108 and thereturn electrode 110, however, may create an undesirable current path202 from the active electrode 104 to the return electrode 110 thatincludes unintended portions 204 of the patient 118. Specifically, asshown in FIG. 2, the combined impedance of the low impedance path 112,the conductive body 108 and the unintended portions 204 of the patient118 is low enough to attract a harmful level of current through theundesirable current path 202.

As a consequence, in accordance with several embodiments of the presentinvention, undesirable current that would otherwise flow in theundesirable current path 202 is limited so as to prevent undesirablecurrent from harming the patient 202. In other words, the undesirablecurrent that would otherwise flow from the active electrode 104 to thereturn electrode 110 through the patient 118, conductive body 108 andthe low impedance path 112 is limited.

FIG. 3 depicts one embodiment of the present invention that limits theundesirable current that would otherwise flow in the undesirable currentpath 202 described with reference to FIG. 2. In the exemplary embodimentdepicted in FIG. 3, an insulator 302 is interposed between theconductive body 308 and the patient 118 so as to limit the amount ofcurrent that may flow from the active electrode 104 in the undesirablepath 202, which includes the patient 118, the conductive body 308 andthe low impedance path 312.

In one embodiment where the conductive body 308 is part of an endoscopefor example, the insulator 302 is an insulating sheath that is added tothe endoscope so as to be interposed between the patient 118 and theconductive body 308 during an electrosurgical procedure.

In another embodiment, the undesirable current is limited by limiting adifference between a voltage of the conductive body 108, 308 and avoltage of the patient 118. Referring to FIG. 4, for example, shown isan exemplary embodiment in which a conductive body 408 is coupled to areference potential 420 via a low impedance path 412. In the exemplaryembodiment the reference potential 420 has a voltage that is establishedso as to render the voltage of the conductive body 408 to besubstantially the same as the voltage of the patient 118. In this way,any currents that do travel from the active electrode 104, through thepatient 118 to the conductive body 418 are much less likely to causedamage to tissues of the patient 118.

Advantageously, the exemplary configuration depicted in FIG. 4 enablesan electrosurgical instrument to be utilized without insulating exteriorportions of the instrument from the patient. In the context ofresectoscopes, for example, a metallic sheath may be utilized becausethe conductive body 408 has a potential, by virtue of being coupled tothe patient 118 via the low impedance path 412, that is close to thepotential of the patient 118.

As a consequence, manufacturers of resectoscopes need not retool toaccommodate an insulating sheath, and metallic sheaths often times havea longer life span and smaller size than insulating sheaths. Moreover,most surgeons are accustomed to and prefer the look and feel ofstainless steel components.

In some variations of the embodiment depicted in FIG. 4, the referencepotential 420, in connection with the low impedance path 412, maintainsthe conductive body 408 at a potential that is within 25 Volts of thepotential of the patient. In other variations, the potential of theconductive body 408 is maintained to within 15 Volts of the patient. Inyet other variations, the potential of the conductive body 408 ismaintained to within 10 Volts of the patient potential, and inaccordance with still other variations, the reference potential 420 isvaried so as to maintain the potential of the conductive body to within3 Volts of he conductive body 408.

As depicted in FIG. 4, a protective circuit advantageously utilizes aseparate reference potential 420, which lacks a voltage offset (i.e., asubstantially lower voltage than a patient voltage) that is inherentwith a return electrode (e.g., the return electrode 110). As aconsequence, currents that would ordinarily flow from the activeelectrode 104 through an undesirable path that includes the patient 118and the conductive body 408 are substantially reduced or preventedaltogether. Thus, any tissue of the patient 118 or the operator/surgeonthat contacts the conductive body 408 is protected from being a part ofthe undesirable current path.

Moreover, because the conductive body 408 is coupled to the referencepotential 420 (i.e., via the low impedance path 412) instead of thereturn electrode 110, this embodiment is aligned with internationalstandards such as IEC 601-2-2. This in turn may allow the use of ametallic sheath as a alternate to an insulated sheath. This is desirablebecause it comports with the user's customary instruments, it isdurable, and aids in achieving a minimum instrument diameter.

It should be recognized that the embodiments described with reference toFIG. 4 are certainly not limited to applications involving endoscopes.For example, coupling the conductive body (e.g., working element) of avariety of electrosurgical devices (e.g., colonoscopes and cathetersystems) to the reference potential 420 via the low impedance path 412is advantageous for one or more of the reasons discussed above.

Although embodiments described with reference to FIG. 4 do haveadvantages over embodiments described with reference to FIG. 3, itshould be recognized that the embodiments described with reference toFIG. 3 do provide a viable approach to improving the safety ofelectrosurgical procedures.

While referring to FIGS. 3 and 4, simultaneous reference will be made toFIG. 5, which is a flowchart depicting steps traversed in accordancewith one method for performing an electrosurgical procedure. As shown,the active electrode 104 is initially applied to the patient 118 alongwith the return electrode 110 so as to create a current path 206 intissue of the patient 118 between the active electrode 104 and thereturn electrode 110 (Blocks 500-506).

In addition, the conductive body 308, 408 is coupled to a referencevoltage with a low impedance path. (Block 508). In the embodimentdepicted in FIG. 3, the reference voltage is the voltage of the returnelectrode 110, and in the embodiment of FIG. 4, the reference voltage isthe voltage of the reference potential 420.

As shown in FIG. 5, a voltage is then imparted to the active electrode104 so as to generate current in the current path 206 that alters tissueof the patient 118 (Block 510). While the voltage is imparted to theactive electrode 104, any undesirable current 202 that would otherwiseflow from the active electrode 104 to the reference voltage 110, 420through the patient 118, conductive body 308, 408 and low impedance path312, 412 is limited (Block 512).

Additionally, in several embodiments, a non-zero level of conductionbetween the conductive body 308, 408 and the active electrode 104 ismonitored while continuing to impart the voltage to the active electrode104 (Block 514), and the voltage of the active electrode 104 is alteredin response to a particular variation of the non-zero level ofconduction between the conductive body 308, 408 and the active electrode104 (Blocks 516, 518).

In many embodiments, variations in the level of conduction between theactive electrode 104 and the conductive body 308, 408 are tolerated forone or more periods of time. For example, when the electrosurgicalprocedure depicted in FIG. 5 is carried out with either a resectoscopeor hysteroscope, chips of tissue and/or blood can cause a temporaryconduction between the active electrode 104 and the conductive body 308,408. Although the patient 118 and operator may need protection from thiscondition, in these embodiments, such a temporary conduction is not afault condition per se that requires the generator 302, 402 to be shutdown completely. As a consequence, the monitor 314, 414 in someembodiments responds to the temporary conduction with a signal 316, 416that does not immediately shut down the generator.

For example, in some embodiments the monitor 314, 414 provides a warningto the operator without initiating a reduction of power to activeelectrode. In other embodiments, the particular variation that causes analteration of the current level between the active electrode 104 and theconductive body 308, 408 is a particular current level that is sustainedfor a predetermined amount of time. The alteration to the voltageimparted to the active electrode 104 in some embodiments is a reductionin the voltage applied to the active electrode 104 so that the energylevel is brought to a level that the patient's body can tolerate withoutharm. In yet other embodiments, the alteration to the voltage impartedto the active electrode 104 is a complete removal of the voltageimparted to the active electrode.

In several embodiments, the monitoring for any conduction between theconductive body 308, 408 and the active electrode 104 is carried out bymetering a parameter that has a value that varies with the conductionbetween the conductive body 308, 408 and the active electrode 104. Insome embodiments for example, the conduction between the conductive body308, 408 and the active electrode 104 is carried out by metering thelevel of current in the low impedance path 312, 412. In otherembodiments, the monitoring for conduction between the conductive body308, 408 and the active electrode 104 is carried out by metering avoltage of the conductive body 308, 408. In these embodiments, themonitoring includes an indirect measurement of the current flowingbetween the active electrode 104 and the conductive body 308, 408.

The reference potential 420 in some embodiments is generated based upona voltage known to be close to a typical human voltage. In otherembodiments, the reference potential is derived from at least onephysical characteristic of the patient.

Referring next to FIG. 6, for example, shown is a block diagram 600depicting one embodiment in which the reference potential 420 is deriveddirectly from a voltage of the patient. As depicted in FIG. 6, aconductive body 608 is connected through a monitor 614 to a referencepotential electrode (RPE) 640. The RPE 640 in this embodiment ismaintained at a reference potential that is consistent with the apotential of the patient's 618 body. In this embodiment, the couplingbetween the conductive body 608 and the RPE 640, which includes themonitor 614, creates a low impedance path (e.g., less than 100 Ohms)from the conductive body 608 to a reference potential 640, which in thisembodiment, is obtained from a direct coupling of the RPE 640 to thepatient 118.

The RPE 640 in some embodiments is realized as a completely separateelectrode that is coupled to an opposite or alternate site of thepatient 118, and in other embodiments the RPE 640 is implemented as aseparate conductive area or areas in a return electrode assembly. Inboth of theses types of implementations, the RPE 640 and the returnelectrode 610 each have a separate contact area on the patient 118 thatelectrically isolates, to a substantial degree, the RPE 640 and returnelectrode 610. Exemplary electrodes that are suitable for implementationas either the return electrode 610 or the RPE 640 are disclosed in U.S.Pat. Nos. 4,416,276 or 4,416,277, the details of which are herebyincorporated by reference into the present application.

Referring next to FIG. 7, shown is a block diagram depicting anexemplary electrosurgical system 700, which is configured to generate aderived reference. As depicted in FIG. 7, a processor 702 is coupled toa current sensor 704, a dependent voltage source 706, a currenttransducer 708, an external input 710 and a contact quality monitor 712.Also shown is a monitor 714 that is coupled to a conductive body 716, anRF power source 718, and via a derived reference line 715, to thedependent voltage source 706. The conductive body 716 in this embodimentis a working element that forms part of an endoscope 717, which includesa telescope 718, a tube assembly 720 and an active electrode 722. Asdepicted in FIG. 7, the active electrode 722 is in contact with thepatient 118. In addition, a reference electrode 724 and return electrode726 are shown coupled to the patient 118 at different locations of thepatient 118. The reference electrode 724 is shown coupled to adifferential voltage transducer 728 and the return electrode 726 isshown coupled to a current sensor 704 (e.g., via inductive coupling).

The processor 702 in several embodiments includes analog and digitalcomponents and a variable gain amplifier (not shown). One of ordinaryskill in the art will recognize, however, that the processor 702 may berealized in other embodiments as an entirely analog or entirely digitalprocessor and may be one integrated processor (e.g., an ASIC or PICcontroller) or several discrete components. In the present embodiment,the analog and digital components control the variable gain amplifier soas to provide an output 730 to the dependent voltage source 706, whichaffects the derived reference voltage 715 that is generated by thedependent voltage source 706.

The output 730 of the processor, and hence, the derived referencevoltage 715 of the dependent voltage source 706 is a function of one ormore of the inputs 732 to the processor 702. In particular, theprocessor 702 receives a signal 734 from the current sensor 704 (e.g., acurrent transducer), which is indicative of a level of current in thereturn line 726. The processor 702 then scales the signal 734 from thecurrent sensor 704 as a function of other inputs 736, 740, 742 to theprocessor 702. In several embodiments, the processor 702 continuouslyreceives the inputs 732 and adapts the output 730 to the changingconditions/of the patient 118.

As depicted in FIG. 7, one of the inputs 732 to the processor 702 is asignal 736 from the contact quality monitor 712, which is indicative ofan impedance of the patient 118 at a location 738 where the returnelectrode 726 contacts the patient. In this embodiment, the returnelectrode 726 includes two return wires (not shown), and each of thereturn wires is separately coupled to the patient 738. The contactquality monitor 712 in the present embodiment meters an impedance of thepatient 118 between the two return wires, and provides the signal 736 tothe processor 702.

Another input to the processor 702 in the exemplary embodiment is theexternal input 710. Although the external input 710 is depicted as asingle line for simplicity, in some embodiments the external input isrealized by multiple inputs to the processor 702. In this embodiment,the external input is a signal 740 that is indicative of one or morevariables such as an amount of body fat in the tissue of the patient118, the particular portion of the patient 118 being operated upon andinformation about the locations on the patient 118 where the electrodes724, 726 are being placed. These factors are merely exemplary, however,and other factors may be utilized by the processor 702 as well.

Yet another input to the processor 702 in the exemplary embodiment is asignal 742, which is indicative of a difference between the voltage ofthe return electrode 726 and a voltage of the reference electrode 724(i.e., a voltage of the patient 118 at the location 744 where thereference electrode 724 is coupled to the patient 118). In someembodiments, the reference electrode 724 is part of an electrodeassembly that includes the return electrode 726, but this is certainlynot required, and in other embodiments the return electrode 726 andreference electrode 724 are completely separated.

As depicted in FIG. 7, the differential voltage transducer 728 generatesan output 742 that is proportional to the difference between the returnelectrode 726 and the reference electrode 724. Although depicted as asingle functional block, the differential voltage transducer 728includes a an RMS responding detector that generates the output 742.

In the exemplary embodiment, the dependent voltage source 706 is anisolated amplifier with a differential output 715 that is a function ofthe output 730 of the processor 702 relative to the voltage of thereturn electrode 726. In this embodiment, the processor 702 provides theoutput signal 730 at a level that prompts the dependent voltage source706 to generate, as the derived reference 715, a voltage between 0 and50 Volts RMS referred to the voltage of the return line 726.

As shown in FIG. 7, the monitor 714 in this embodiment couples theconductive body 716 to the derived reference 715. The monitor 714 inseveral embodiments is a low impedance monitor (e.g., less than 100Ohms) so as to provide a low impedance path 709 between the conductivebody 716 and the derived reference 715. In one embodiment, for example,the monitor is an EM-2 style monitor manufactured by Encision, Inc ofBoulder Colo. As shown, a current transducer 708 is configured to sensea level of current in the low impedance path 709 and provide an output750 to the processor 702 that is indicative of the level of current inthe low impedance path 709. One of ordinary skill in the art willrecognize that the current transducer 708 may be realized by a varietyof current transducers.

In some embodiments, the processor 702 generates the output 730 at alevel that translates to a derived reference voltage 715 that issubstantially the same as a voltage of the patient 118 at the surgicalsite 746. In this way, the voltage of the conductive body 716 relativeto the surgical site of the patient 746 is limited to a relatively smallvalue (e.g., less than 25 Volts) that is a function of the current inthe low impedance path 709 from the conductive body 716, through themonitor 714, to the derived reference 715.

In other embodiments, the processor 702 generates the output 730 at alevel that translates to a derived reference 715 that compensates forcurrent flow in the low impedance path 709 from the conductive body 716through the monitor 714 to the derived reference 715 so as to render thevoltage of the conductive body 716 at a level that is substantially thesame as a voltage of the patient 118 at the surgical site 746.

As shown in FIG. 7 for example, in the event the current level from theconductive body 716, through the monitor 714, to the derived reference715 increases (indicating a voltage of the conductive body 716 is higherthan the patient voltage), the current transducer 708 provides theoutput 750 to the processor 702 at a level that is indicative of theincreased level of current in the monitor 714. In turn, the processor702 adjusts the output signal 730 to the dependent voltage source 706 sothat the derived reference voltage 715 is decreased. In this way, thevoltage of the conductive body 716 is also reduced back to the level ofthe patient at the surgical site 746.

Referring next to FIG. 8, shown is a block diagram depicting anotherembodiment of an electrosurgical system 800, which is configured togenerate a derived reference 815 utilizing a reference potentialelectrode 850. The electrosurgical system 800 operates in a similarmanner as the system 700 depicted in FIG. 7 except the derived reference815 in the present embodiment is referenced to a potential of thepatient 860 at the reference potential electrode 850 instead of thereturn electrode 726. In addition, a contact quality monitor 870 in thisembodiment provides a signal 836 which is indicative of an impedance ofthe patient 118 at a location 860 where the reference potentialelectrode 850 contacts the patient 118. The differential voltagetransducer 728 in this embodiment generates an output 842 that isproportional to the difference between the reference potential electrode850 and the reference electrode 724.

As depicted in FIG. 8, the processor 802 receives and scales the signal842 from the differential voltage sensor 728 as a function of otherinputs 836, 740, 850 so as to generate an output 830 which is convertedto the derived reference voltage 815 by the dependent voltage source706. In several embodiments, the processor 802 continuously receives theinputs 832 and adapts the output 830 to the changing conditions/of thepatient 118.

Referring next to FIG. 9, shown is a flowchart 900 depicting stepscarried out to prepare an electrosurgical instrument for anelectrosurgical procedure in accordance with the exemplaryelectrosurgical systems of FIGS. 7 and 8. In operation, the processor702, 802 initially receives information indicative of at least onephysical characteristic of the patient 118 (Blocks 902, 904).

As depicted in the exemplary embodiments of FIGS. 7 and 8 and discussedabove, the processor 702, 802 is configured to receive informationindicative of different physical characteristics of the patient from theexternal input 710, the contact quality monitor 712, 870 and the currenttransducer 708. The information from the external input 710 may includean indication of fat content in the tissue of the patient, theparticular portion of the patient 118 being operated upon andinformation about the locations on the patient 118 where the electrodes724, 726, 826, 850 are being placed. The signal 736, 836 from thecontact quality monitor 712, 870 is indicative of an impedance of thepatient 118 at a location 738, 860 where the return electrode 726, 826contacts the patient 118. In the embodiment depicted in FIG. 7, theoutput 742 of the voltage transducer 728 is indicative of a differencebetween the voltage of the return electrode 726 and a voltage of thepatient 118 at the location 744 where the reference electrode 724 iscoupled to the patient 118. In the alternative embodiment depicted inFIG. 8, the output 842 of the voltage transducer 728 is indicative of adifference between the voltage of the reference potential electrode 850and a voltage of the patient 118 at the location 744 where the referenceelectrode 724 is coupled to the patient 118.

The processor 702, 802 in connection with the dependent voltage source706, then generates a reference voltage (e.g., the derived reference715, 815) based upon at least one of the physical characteristics of thepatient (Block 906). In the embodiment discussed with reference to FIG.7, the current signal 734 is scaled by a function that includes, asinputs, the signals 736, 740, 742 from the contact quality monitor 712,the external input 710 and the detector 708, respectively. In thealternative embodiment discussed with reference to FIG. 8, the processor802 receives and scales the signal 842 from the differential voltagesensor 728 as a function of other inputs 836, 740, 850 so as to generatean output 830 which is converted to the derived reference voltage 815.

The reference voltage (e.g., the derived reference 715, 815) is thencoupled to a conductive body (e.g., the working element 716) of theelectrosurgical apparatus (e.g., the endoscope 717) so as to limit anydifference between the voltage of the surgical site 746 of the patient118 and the body of the electrosurgical apparatus (e.g., theresectoscope 717) (Blocks 908, 910).

In the embodiments depicted in FIGS. 7 and 8, the monitor 714, 814provides additional safety by monitoring current flow between theconductive body 716 (e.g., a working element) of the electrosurgicalapparatus and the active electrode 722 and alters the level of voltageprovided to the active electrode 722 by sending a control signal 748,848 to the power source 718. In some embodiments, the alteration of thevoltage that is applied to the active electrode 722 may be a modulationof the active electrode voltage as a function of one or morecharacteristics of the current monitored between the active electrode722 and a conductive body 716 of the electrosurgical instrument (e.g.,the resectoscope 717).

As discussed above, in the context of endoscopes, some current isexpected to flow between the active electrode 722 and the conductivebody 716 while an electrosurgical procedure is being carried out (e.g.,due to conductive tissue and/or fluid that becomes interposed betweenthe active electrode 722 and the conductive body 716). As a consequence,in some embodiments, the monitor 714 sends the control signal 748, 848at a level that directs the power source 718 to continue to impart avoltage to the active electrode 722, for at least an acceptable periodof time, while there is a non-zero level of conduction between theactive electrode 722 and the conductive body 716. In this way, theelectrosurgical procedure is not interrupted due to the expectedconduction between the active electrode 722 and the conductive body 716.

Although many variations of the system depicted in FIGS. 7 and 8 aredescribed herein within the context of procedures performed utilizingendoscopes, it is contemplated that generating the derived reference 715and coupling the derived reference 715 to a conductive body of any oneof a variety of electrosurgical devices provides a substantial level ofsafety by limiting a level of voltage that the body of theelectrosurgical devices may attain. It should also be recognized thatneither any one nor all of the inputs 732, 832 must be utilized whengenerating the derived reference 715, 815.

Referring next to FIG. 10, shown is a cross sectional view of anexemplary electrode assembly structure 1000, which may be utilized inany of the embodiments described with reference to FIGS. 1-9. As shown,an active electrode 1002 is surrounded in part by an insulator layer1004, a shield 1006 and an outer insulator 1008 that are stacked in aradial direction relative to the active electrode 1002. The activeelectrode 1002 in several embodiments is custom designed forimplementation as part of the assembly structure 1000. The insulator1004 in the exemplary embodiment may be composed of a variety ofplastics including polyaryletheretherketone (e.g., sold under thePEEK.TM. brand) and fiber reinforced polymer.

The shield 1006 in this embodiment is a conductive material that mayinclude stainless steel and/or aluminum. As depicted in FIG. 10, theshield is arranged so as to protect the insulator 1004 that surroundsthe active electrode 1002 from being pierced. In this way, the shieldhelps to maintain electrical isolation between a conductive body (e.g.,working element) of the electrosurgical device (e.g., an endoscope) thatemploys the electrode assembly 1000.

In some embodiments, as discussed further with reference to FIG. 11, theshield 1006 is adapted so as to be capable of being conductively coupledto a monitor, and the monitor is then able to assess the integrity ofthe electrode assembly by monitoring a level of conduction between theshield 1006 and the active electrode 1002.

As shown, the outer insulator 1008 is disposed so as to insulate theshield 1006 from other components of the electrosurgical device when theelectrode assembly 1000 is installed and utilized. The outer insulator1008 may be realized by similar materials as the inner insulator 1004,but the outer insulator need not have the level of strength nor the lowdielectric constant of the inner insulator 1004.

Referring next to FIG. 11, shown is a block diagram of a system 1100 formonitoring both a conductive body 1108 and a shield 1120 of an electrodeassembly (e.g., the electrode assembly 1000) during an electrosurgicalprocedure. In this embodiment the active electrode 1104 incorporates theactive-insulation-shield-insulation construction, described withreference to FIG. 10, through an otherwise conventional conductive body1108.

As depicted in FIG. 11, an RPE 1140 is utilized to provide a referencepotential that is derived from a direct coupling of the RPE 1140 to thepatient 1118. In alternative embodiments, the conductive body 1108 iscoupled to a derived reference (e.g., the derived reference 715) that isgenerated based upon one or more physical characteristics of thepatient. The RPE 1140 in the present embodiment may be a completelyseparate electrode on an alternate site of the patient 118, or it may bea partitioned area in a return electrode assembly.

In this embodiment, the shield component 1120 of the electrode assembly(not shown) is returned to a return electrode connection of thegenerator 1102 through the monitor 1114 in a manner that is similar toprior AEM laparoscopic instruments described, for example, in the Newton'401 patent.

The conductive body 1108 in this embodiment is connected to thereference potential electrode (RPE) 1140 as described above, and themonitor 1114 in the exemplary embodiment includes two separate channels.A first channel 1160 monitors the current flowing from the shield 1120to the return electrode 1110, and the first channel is configured toalter the power output from the generator 1102 by sending a controlsignal 1116 to the generator 1102 in the event of a fault condition. Insome embodiments, the channel 1160 does not distinguish between a normaland abnormal fault condition, and instead, it simply shuts off the powerif a fault condition is detected. A second channel 1170 monitorscurrents between the conductive body 1108 and the RPE 1140 and alterspower imparted to the active electrode 1104 by inhibiting and/orreducing power as described with reference to other embodiments depictedin FIGS. 3-9.

In this embodiment, large currents flowing through an insulation failureof the electrode assembly have a different path than smaller currentsflowing through the conductive body 1108, with different monitoringthresholds and monitoring effects. For example, the monitor 1114 mayhave a fixed current threshold, a fault current threshold proportionalto the active current, and/or the monitor 1114 may produce a warningwhen the threshold is exceeded.

Alternatively, the monitor 1114 may have two thresholds that include alower threshold that triggers a warning and a higher threshold thattriggers a signal 1116 from the monitor 114 to the generator 1102 thatreduces power to the active electrode 1104. As previously discussed,fault currents in the conductive body 1108 can be temporarily induceddue to tissue or conductive fluid that causes coupling between theactive electrode 1104 and the conductive body 1108. Under such a faultcondition, a warning rather than an alteration of the power isadvantageous. Another advantage of this configuration is that insulationfault currents will have a direct path to the return electrode 1110 anddo not challenge the path involving the RPE 1140 and the conductive body1108.

Referring next to FIG. 12, shown is a schematic representation of aresectoscope 1200 that may be used in the embodiments disclosed withreference to FIGS. 3-9. As shown, a working element 1202 (i.e., theconductive body of the resectoscope 1200) is coupled to a scope 1204, aninner tube 1206, an outer sheath 1208 and a connector block 1210. Asdepicted in FIG. 12. the connector block is coupled to an active line1212, a first shield lead 1214 and a second shield lead 1216.

Also shown is an electrode assembly 1218, which includes an activeelectrode 1224 with a first end 1226 that is configured to impart avoltage to a region of a patient and a second end 1228 that isconfigured to detachably couple to the active line 1212 of the connectorblock 1210. As shown, portions of the active electrode 1224 between thefirst and second ends 1226, 1228 are surrounded by insulation 1230, andportions of the insulation 1230 are surrounded by a shield 1232, whichis detachably coupled to the first and second shield leads 1214, 1216.In several embodiments, a highly conductive material (e.g., goldplating) is employed at the respective interfaces between the shield1232 and the first and second leads 1214, 1216 and between the activeelectrode 1224 and the active line 1212. The insulation 1230 in theexemplary embodiment may be composed of a variety of plastics includingpolyaryletheretherketone (e.g., sold under the PEEK.TM. brand) and fiberreinforced polymer. The shield 1232 in this embodiment is a conductivematerial that may include stainless steel and/or aluminum.

In this configuration, the connector block 1210 is added to the workingelement 1202 of a standard resectoscopic device in order to provideconductive connections to the working element 1202 and the shield 2332of the electrode assembly 1218. In the exemplary embodiment, the firstand second shield leads 1214, 1216 are both disposed so as to bedetachably coupled with different portions of the shield 1232. The twoshield leads 1214, 1216 provide a redundant, and hence more reliable,coupling to the shield 1232. In addition, the two leads 1214, 1216enable the connection between the shield leads 1214, 1216 and the shield1232 to be tested by measuring the continuity between the shield leads1214, 1216. In this way, when the active electrode assembly 1218 isinserted into the resectoscope 1200, a simple continuity test ensuresthe electrode assembly is properly engaged with the resectoscope 1200.

As depicted in FIG. 12, the second shield lead 1216 in this embodimentis coupled to the working element 1202 so as to conductively couple theshield 1232 and the working element 1202. In this way, both the shield1232 and the working element 1302 may be conveniently coupled to areference potential (e.g., the reference potential 420) via a monitor(e.g., the monitor 114, 214, 314, 414, 614, 714 and 1014).

In this embodiment, the inner tube 1206 and outer sheath 1208 are alsocoupled to the working element 1202 so that the working element 1202,the shield 1232, the inner tube 1206 and the outer tube 1208 havesubstantially the same voltage. Both the inner tube 1206 and outersheath 1208 have a low resistance conduction to the working element1202. A gold plating, or other good conductor, are preferentialsolutions for this purpose.

The connector block 1210 also provides a connection between the activeelectrode 1224 and the active electrode lead 1212. In this embodiment,the working element 1202, scope 1204 and inner tube 1206 are metallic.The outer sheath 1208, however, is metallic in some variations and is aninsulator (e.g., fiber reinforced plastic) in other variations.

In some variations of the embodiment depicted in FIG. 12, the secondshield lead 1216 is disconnected from the working element 1202 and aseparate lead to the working element 1202 is provided within theconnector block so as to enable both the shield 1232 and the workingelement 1202 to be coupled to the separate channels of the dual channelmonitor 1114 described with reference to FIG. 11. In yet othervariations, the electrode assembly 1000 described with reference to FIG.10 may be employed in the resectoscope depicted in FIG. 12.

Referring next to FIGS. 13A, 13B and 13C, shown are a front, a top and across sectional view of an exemplary embodiment of a resectoscope 1300.As shown in FIGS. 13A and 13B, a working element 1308 is coupled to aconnector block 1310 and an outer tube assembly 1312. In FIG. 13C, whichis a cross-sectional view of the resectoscope 1300 taken along sectionJ-J of FIG. 13B, shown is a telescope portion 1316 of the resectoscope1300 within the working element 1308 and the outer tube assembly 1312.

Referring to FIG. 14, shown is a perspective view of the resectoscopedepicted in FIG. 13 in a disassembled form. As shown, the telescopeassembly 1316 is configured to fit within the working element 1308, andthe electrode assembly 1304 is configured to couple to an exteriorportion of the working element 1308 so as to be able to move relative tothe working element 1308. Also shown is an inner tube assembly 1318 thatis configured to slide over both the electrode assembly 1304 and theworking element 1308. In addition, the outer tube assembly 1312 isconfigured to slide over the inner tube assembly 1318.

Referring next to FIGS. 15A and 15B, shown are a cross sectional and afront view of a portion of the resectoscope depicted in FIGS. 13 and 14.As shown in FIGS. 15A and 15B, the telescope 1316 and active electrodeassembly 1304 fit within the inner tube assembly 1318 while providingsufficient space for the inflow of irrigating fluid.

In conclusion, the present invention provides, among other things, asystem and method for monitoring, and rendering safer, electrosurgicalprocedures. Those skilled in the art can readily recognize that numerousvariations and substitutions may be made in the invention, its use andits configuration to achieve substantially the same results as achievedby the embodiments described herein. For example, many of theembodiments described herein are generally applicable to a range ofelectrosurgical procedures and devices. Accordingly, there is nointention to limit the invention to the disclosed exemplary forms. Manyvariations, modifications and alternative constructions fall within thescope and spirit of the disclosed invention as expressed in the claims.

What is claimed is:
 1. An electrosurgical monitoring apparatuscomprising: at least two monitoring channels, each of the monitoringchannels configured to monitor fault current along a separate fault pathbetween at least two separate conductive components of anelectrosurgical system, wherein a fault path is an unintended currentpath; wherein the at least two monitoring channels are each configuredto send a control signal so as to control an application of power to anelectrosurgical device responsive to the fault currents.
 2. Theelectrosurgical monitoring apparatus of claim 1, wherein at least two ofsaid at least two monitoring channels are configured to monitor thefault currents with different monitoring thresholds.
 3. The monitoringapparatus of claim 1, including derived reference components, thederived reference components configured to apply a potential to at leastone of the at least two monitoring channels that is substantially thesame as the patient.
 4. The electrosurgical monitoring apparatus ofclaim 1, wherein the at least two monitoring channels are housed in aseparate housing from a generator that is controlled by the controlsignal.
 5. The electrosurgical monitoring apparatus of claim 1, whereinthe at least two monitoring channels are implemented in a housing thatis separate from the electrosurgical device.
 6. The electrosurgicalmonitoring apparatus of claim 1, wherein at least one of the at leasttwo monitoring channels shunts current away from the conductive body toa return electrode.
 7. The electrosurgical monitoring apparatus of claim1, wherein at least one of the monitoring channels is configured tomaintain one of the conductive components at substantially the samepotential as the patient.
 8. The electrosurgical monitoring apparatus ofclaim 1, wherein the at least two monitoring channels monitor faultcurrents by sensing current or by sensing voltage of the conductivecomponents.
 9. The electrosurgical monitoring apparatus of claim 1,wherein the electrosurgical device is selected from the group consistingof endoscopes, colonoscopies, resectoscopes, laproscopic instruments andcatheter instruments.
 10. A method for controlling an electrosurgicalsystem, comprising: monitoring at least two fault currents that mayoccur between conductive components of the electrosurgical system,wherein the fault currents each comprise separate unintended currentpaths; controlling a generator based upon the plurality of faultcurrents; and rendering at least one of the conductive components of theelectrosurgical system substantially the same potential as a patient.11. The method of claim 10, wherein the controlling includes controllingthe generator based upon different thresholds for at least two of theplurality of fault currents.
 12. The method of claim 10, wherein themonitoring the at least two fault currents includes monitoring the atleast two fault currents by sensing current or by sensing voltage of theconductive components.
 13. The method of claim 10, wherein the renderingat least one of the conductive components of the electrosurgical systemsubstantially the same potential as a patient includes generating areference voltage that is derived from a characteristic of the patient.14. The method of claim 10, wherein the monitoring the at least twofault currents includes monitoring fault currents between an activeelectrode and at least one of a conductive body of an electrosurgicalinstrument and a shield of an active electrode assembly.
 15. The methodof claim 10, wherein conductive components include an active electrode,a shield of an active electrode assembly, and a conductive body that isnot intended to impart a surgical level voltage to the patient but issusceptible to contacting the patient.
 16. An electrosurgical system,comprising: a first conductive surgical system component; a secondconductive surgical system component; an electrosurgical tool; a monitorincluding at least two monitoring channels, a first of the monitoringchannels monitors fault current flowing through the first conductivesurgical system component, and a second of the monitoring channelsmonitors fault current flowing through the second conductive surgicalsystem component, the fault currents traveling along separate faultpaths, where a fault path is an unintended current path, and the atleast two monitoring channels are each configured to send a controlsignal so as to control an application of power to the electrosurgicaltool responsive to the corresponding fault current.
 17. The system ofclaim 16, wherein the first conductive surgical component includes ashield that protects a portion of the electrosurgical tool.
 18. Thesystem of claim 16, wherein the first conductive surgical componentincludes a conductive body that supports the electrosurgical tool. 19.The system of claim 16, wherein two of the at least two monitoringchannels are configured to monitor the fault currents with differentmonitoring thresholds.
 20. The system of claim 16, including a derivedreference component that provides a reference voltage to at least one ofsaid at least two monitoring channels.